It is known to provide instruments such as wristwatches with a light source for illuminating the dial. The wristwatch dial may be an analog wristwatch dial having indicia or markers used in connection with hands of the watch, or the dial may comprise an electro-optic display of actuated indicia, such as a liquid crystal display. One objective in a dial illumination system has been to achieve a constant and uniform lighting for the dial. Uniform background lighting may be achieved by using a distributed uniform light source, such as an electroluminescent light source. However, in order to reduce the cost of an illumination system for a dial, concentrated light sources, such as a single incandescent bulb or a light emitting diode (LED) have also been considered. One problem then, with using a concentrated light source is to transfer and diffuse the light from a point source of bright light to a uniformly distributed light source for illuminating the dial. The prior art contains many examples of efforts to effect the transformation.
In addition to illumination, light may be used to achieve an aesthetic graphic presentation. In this case it must be possible to locate areas of greater or lesser brightness or different colors at different locations on the dial. Traditional constructions using edge lighting around the outside periphery of a light guide are ineffective in directing the light to the desired location.
Constructions are known for using a light guide behind a light transmissive dial, and to inject light into the light guide. The following U.S. Patents are representative of prior art constructions for illuminating dials from a concentrated light source using a light guide.
______________________________________ U.S. PAT. NO. INVENTOR ISSUE DATE TITLE ______________________________________ 3,748,456 Brien July 24, 1973 Illuminated Instrument Dials 2,140,972 Rylsky Dec. 20, 1938 Means for Illuminating Dials of Instruments 2,188,821 Rylsky Jan. 30, 1940 Compass 2,480,393 Bossert et al. Aug. 30, 1949 Dial Light 3,043,038 Marble July 10, 1962 Illuminated Dial Face 3,561,145 Shotwell Feb. 9, 1971 Light Distributing Lens System ______________________________________
The aforementioned Marble U.S. Pat. No. 3,043,038 and Shotwell U.S. Pat. No. 3,561,145 employ light guides bounded by substantially planar surfaces and attempt to distribute the light rays which are undergoing multiple internal reflections which are introduced from a concentrated source at the peripheral edge of the light guide. These constructions employ tapered cross sections or non-uniformly distributed indentations in order to uniformly distribute the light.
U.S. Pat. No. 5,667,289 issued Sep. 16, 1997 to Akahane et al. describes a background lighting apparatus for a liquid crystal display, in which a non-uniform pattern of light dispersing reflective layers are applied to a light guide illuminated from the edges of the light guide. The reflective layers are arranged to increase in size as the distance from the lighted edge becomes greater.
It is particularly difficult to conduct light from a concentrated source to a light guide in a wristwatch from the edge of a light guide disposed beneath the watch dial. Space is at a premium and the previously known constructions have not been satisfactory.
Accordingly, one object of the present invention is to provide an improved dial illumination system which effects uniform distribution of the light to illuminate a dial.
Another object of the invention is to provide an improved construction for conducting light from a concentrated or point light source to a light guide for illuminating a dial.
Another object of the invention is to provide a construction for directing lighted areas of varying brightness and/or colors in a watch dial illumination system.
Another object of the invention is to provide an improved dial illumination system for distributing light to a wristwatch dial.